The present invention relates to methods and apparatus for protecting digital content from unauthorized copying and for the detection thereof, and is particularly applicable when the digital content to be protected is in the form of a digital movie.
The increased use of digital content, particularly to provide digital movies, often referred to generally as digital cinema, has resulted in an increase in interest in protecting digital content from unauthorized copying. Such protection is particularly useful in digital cinema applications. Digital content can be protected by encryption only up to the moment of presentation, where it becomes vulnerable to unauthorized usage. Embedding digital watermarks in digital content is one known method for protecting such presentations from unauthorized use. In the case of media content watermarking, it is possible to hide some information imperceptibly in media content so as to provide information for determining, for example, the copyright holder, usage rules, date, time, and place of post-decryption theft, such as the use of camcorders in theaters. Visible watermarking may also be provided by, for example, overlaying visual patterns on digital images depending upon the particular requirements for protection and detection of unauthorized copying.
In particular, and with respect to digital cinema, the use of digital content may improve the film industry by transforming the nature of production, delivery and exhibition; by reducing distributor costs; by reducing the lure of piracy due to region-based market delay; and by making it easier for exhibitors to offer alternative content. However, many issues exist to ensure the protection of such digital content from unauthorized copying, and include, for example, secure distribution and display of the movie in digital form. Whether a digital movie is distributed as a set of discs, by means of satellite, or via other forms of networks, unauthorized and/or undetected copying of digital content is a significant risk. For example, first-run movies might be copied without permission for home viewing at the very onset of the theatrical release.
Methods that embed small amounts of information in media may not be suitable for on-the-fly digital watermarking such as that used for digital cinema content protection. Unauthorized analog on-the-fly copies, such as copies of digital cinema movies made by a hand-held camcorder, suffer severe distortion. This distortion may destroy the embedded watermark and inhibits the prevention and/or tracking of digitally-reproduced motion pictures that are recorded, for example, in a digital theater. Moreover, many kinds of watermarks are vulnerable to intentional attacks. Thus, digital watermarking alone can only provide a moderate level of security by means of tracking illegal distribution.
A number of data hiding techniques are known in the prior art. One known technique for data hiding is known as the “Patch Work” algorithm developed at M.I.T. and described by Gruhl and Bender in “Information Hiding to Foil The Casual Counterfeiter,” at pp. 1–15 in Information Hiding 1998 LNCS 1525, Springer-Verlag, Berlin, which is hereby incorporated by reference in its entirety. This algorithm chooses a number of “patches” and then modifies the patches to change the statistical distribution for watermark embedding. Patch centers are defined in reference to the length and height of an image and a fixed pixel, for example, the [0, 0]th pixel of the image. Although this system is satisfactory for undistorted images, image distortion, such as rotation or nonlinear distortion, will introduce decoding errors.
Many other watermarking algorithms have been proposed. One popular fragile digital watermarking algorithm performs least significant bit modulation to embed a watermark W in the least significant bit (LSB) stream. Although this watermarking algorithm is among the easiest to implement in real time, it can be hindered or defeated by certain types of transformations and signal distortions.
U.S. Pat. No. 5,848,155 to Cox et al., which is hereby incorporated in its entirety by reference, describes a spread spectrum watermarking algorithm. This algorithm forms the basis for some of the most popular robust watermarking algorithms. Although the Cox et al. algorithm and many improved versions thereof can withstand certain types of signal processing noise (such as that add by low pass filtering, re-compression, and white noise addition), it does not fare as well over DA-AD (digital to analog, followed by analog to digital) conversions, geometrical image distortion, and large scale down sampling transformations that occur in digital cinema camcorder copying.
In U.S. Patent Publication No. 20020106103 to Jones, entitled “System and method for embedding a watermark signal that contains message data in a digital image” and which is hereby incorporated by reference in its entirety, there is described a suprathreshold watermarking algorithm that is useful for digital cinema. However, as it is an image-based algorithm, it, too, fares less well when subjected to various types of distortions, particularly geometrical image distortion.